As shown schematically in FIG. 1, a conventional micrometer microscope usually has an optical system comprising a light source Q.sub.1, a filter F.sub.1, a condenser lens C, an objective lens O.sub.1 and a Filar micrometer eyepiece ME. In such a system, a sample S is magnified by the objective lens O.sub.1 and produces a sample image S.sup.1, which is then measured by means of the Filar micrometer eyepiece ME. The value of the measurement thus obtained is divided by the magnification value of said objective lens O.sub.1 to determine the actual dimension of the sample S.
With all the recently increasing demand for an optical instrument of this kind capable of measuring extremely fine dimensions of IC, LSI masks and Wafer patterns with the required precision as well as in large numbers, it has become clear that the major disadvantages of such conventional instruments are:
(1) The calculation of the actual dimension of the sample is effected with ease when the magnification value of the objective lens O.sub.1 is 10 or 100, but it is rather difficult in the case of other values, such as 40 or 60 and the like.
(2) In the event that an objective lens of high magnification is used, the contrast of the sample image is necessarily poor as compared with the clear view of the scales seen directly through the eyepiece ME, making it difficult to align the sample image and the scale lines in such a manner that they precisely overlap each other, therefore resulting in a significant decrease in the precision of the measurement.
(3) Long hours of monocular observation naturally cause more fatigue that does a similar length of time of binocular observation.